1. Field of the Invention
The present invention relates to an integrated thin-film solar cell module and a method of manufacturing the same, and more particularly, to a thin-film solar cell and a method of manufacturing the same, which are capable of improving a photoelectric conversion efficiency.
2. Discussion of the Related Art
The research in solar cells as next generation clean energy resources has been carried out for many years. Until recently, as materials for solar cells, materials of group IV such as monocrystalline silicon, polycrystalline silicon, amorphous silicon, amorphous SiC, amorphous SiN, amorphous SiCe and amorphous SiSn, and compound semiconductors of group III-V such as gallium arsenide (GaAs), aluminum gallium arsenide (AlGaAs) and indium phosphorus (InP) or group II-VI such as CdS, CdTe and Cu2S have been used. Further, structures for the solar cell include a pn structure including back electrode layer; a pin structure; a heterojunction structure; a schottky structure; and a multiple junction structure including tandem or vertical junction type.
In general, as the characteristics required for the solar cell, there are a high photoelectric conversion efficiency, low production costs and a short energy recovery period.
A solar cell using monocrystalline bulk silicon, which is being commonly used at present, has a high photoelectric conversion efficiency, but the fact is that it is not being actively utilized due to high production and installation costs thereof. Research into thin-film solar cells has been actively carried out to solve such problems. Particularly, a thin-film solar cell using amorphous silicon (a-Si:H), which is capable of reducing production costs of large size solar cell modules and energy recovery period, has drawn more attention. However, the thin-film solar cell has problems such that the photoelectric conversion efficiency thereof is lower than that of a monocrystalline silicon solar cell and the efficiency is decreased when exposed to light.
For a solar cell using the other materials, in a case where the photoelectric conversion efficiency thereof is high, fabrication costs are increased and energy recovery period is extended, and in contrast, low fabrication costs and a short energy recovery period result in a low photoelectric conversion efficiency.
A structure including semiconductor layers having the different band gap energy and a buffer layer formed therebetween has been suggested to improve the above problems such as the low photoelectric conversion efficiency of the thin-film solar cell using amorphous silicon. Particularly, an up-down stack structure of amorphous silicon (a-Si:H) and microcrystalline silicon (uc-Si:H), which have the different band gap energy and crystal lattice mismatch, has been suggested.
FIG. 1 is a sectional view illustrating a stack structure of a thin-film solar cell module according to an example of the prior art.
In the thin-film solar cell module according to the example of the prior art shown in FIG. 1, a first solar cell layer 120 and a second solar cell layer 130 with the different characteristics and crystal structure consist of a stack structure and are electrically connected in series by connecting a transparent conductive layer 111 stacked on the unit cell of the second solar cell 130 with a transparent conductive layer 110 stacked under the adjacent unit cell of the first solar cell layer 120.
FIG. 2 is an equivalent circuit diagram of a diode illustrating a series connection of the semiconductor layers.
In general, a first solar cell layer toward incident sunlight consists of amorphous silicon and has a high band gap energy of about 1.7 to 1.9 eV, whereas, a second solar cell layer stacked onto the first solar cell layer consists of microcrystalline silicon and has a band gap energy of about 1.1 eV. Thus, since the solar cell layers having different absorption bands are stacked, the thin-film solar cell formed of both solar cell layers has a higher photoelectric conversion efficiency as compared with the thin-film solar cell formed of single solar cell layers. As a result of the research, a small module having an area of 3 cm2 produces an initial photoelectric conversion efficiency of about 14.5% and a large area module produces an initial conversion efficiency of about 12%.
However, the solar cell structure on which the different double solar cell layers are stacked causes a problem such that the current of both the solar cell layers should be designed to be equal because both the solar cell layers are connected in series. By such a restriction, the thickness of an amorphous silicon intrinsic semiconductor layer of the first solar cell layer, which is located at the lower part, should be thickly formed beyond what is otherwise needed, and in proportion to the thickness, as the electric power rate generated at the amorphous solar cell layer become high, the total efficiency due to the Stabler-Wronski effect is excessively decreased. Conversely, in the case that the thickness of the intrinsic semiconductor layer is most suitable and thin, a short-circuit current of the first solar cell layer located at a lower part become small, and thus, as the difference of the short-circuit current between both the solar cell layers is increased, the total efficiency of the element in which two layers are connected in series is reduced compared with the total efficiency achieved by both solar cell layers because the whole short-circuit current is limited by the short-circuit current of the first solar cell layer.
In order to get over a difficulty of manufacturing process such that it is not easy to adjust the thickness of intrinsic semiconductor for producing an optimal photoelectric conversion efficiency at solar cells on which the different double solar cell layers are stacked, and in order to establish a stable reliability in a regular efficiency of manufactured solar cells, U.S. publication patent No. 2005/0150542 A1 discloses a solar cell module such that a first solar cell layer 220 and a second solar cell layer 230 are respectively connected to an adjacent cell in series, by separating the first solar cell layer 220 located toward the lower part from the second solar cell layer 230 located toward the upper part by a transparent insulating layer and providing a 4-T structure which draws two terminals from each solar cell layer.
FIG. 3 is a sectional view illustrating a stack structure of a 4-terminal thin-film solar cell module, which is disclosed in the above U.S. patent, and FIG. 4 is a diode equivalent circuit of the 4-terminal thin-film solar cell module.